fetch & build grammars into the source runtime dir in helix-term builder

[the-mikedavis]

Currently the helix-term builder calls out to the same functions as are used in the hx -g fetch and hx -g build CLI options. The CLI options fetch and build grammars into ~/.config/helix/runtime/grammars directory (on unix). This is problematic if one updates their helix installation from source using cargo install and then removes and copies or links the runtime directory from source: the ~/.config/helix/runtime/grammars directory gets deleted and needs to be recreated with manual calls to hx -g fetch and hx -g build.

We should use the source repo's runtime directory when fetching and building grammars in the helix-term builder. Also, as @dead10ck points out on the matrix, it's also not very friendly for a compilation phase to go modify files outside the source directory.

I suspect that #2012 falls under this.

[the-mikedavis]

Fixed by #5411